1. Field of the Invention
The present invention relates to a frequency converter which converts the frequency of a digital signal by inserting a stuff or dummy pulse into a multiplexed signal; more particularly, the present invention is used in converting the frequency of an input signal stored in a memory by a write clock and read from the memory by a read clock having a frequency higher than the write clock frequency.
2. Description of the Related Art
In a communication system using such a frequency converter, the write clock and read clock have different repetition frequencies, therefore the phase difference between write clock and read clock in a transmitter is gradually reduced and the same input signal is sometimes read twice. In order to prevent such rereading, when a phase difference becomes smaller than a constant value, the read operation is inhibited and a stuff or dummy pulse is inserted. Since this dummy pulse is invalid as data, it must be eliminated in the receiver. Accordingly, a stuff designation pulse which indicates existence or nonexistence of the stuff pulse is also inserted together with the stuff pulse, and these signals are transmitted simultaneously.
As described in Japanese Laid-open Patent No. 56-126343, for example, a stuff designation pulse conventionally has a 3-bit structure which allows for transmission error and permits existence of a stuff pulse to be judged on the basis of decision by majority in the receiver. This system has no problem when the repetition frequency of the read clock is sufficiently higher than the write clock, but if the difference between the repetition frequencies of the read clock and write clock is small, the stuff designation pulse cannot be transmitted because there is no margin for inserting a three bit stuff designation pulse.